Its skid plates, a 3.73 Limited Slip or today's trucks are "Locking Rear Differential". Generally it is a center shifter ****, but mine is a FX4 with the shifter on the steer wheeling... it has FX4 gauges. Also the shocks on FX4 are generally different then those on a regular 4x4... Now you can get them pretty much loaded anyway you want... I've seen F250's from the factory come pretty much base but have FX4 stickers on them meaning skid plates and all that.
I think it is a play on Ford and effects 4 being that is mostly a trim package. Like effects in movies are called FX.
I've thought the same. Yes, you get the skid plates and everything, but the part that I like the most is that all FX models, both FX4 and FX2's have NO chrome on them. I'm not a fan of the chrome bumpers and grills. The monochrome look and body matched bumpers are more of a "sport" look. I kind of viewed the FX4 as a "4x4 sport" model. However that doesn't work in any way to "fx4"....
it is also just a 'package' I have an odd truck I bought barely used in 04..a lariat with the 'fx4' package. it was ordered by the first owner and sold to me with 12k on the odometer. it seems to have a bit of both.